1. Field of the Invention
This invention relates to electrical interconnections, and more particularly to a method of forming electrical interconnections across gaps by electroplating.
2. Description of the Prior Art
In the prior art electrical interconnections between miniature components such as integrated circuits and infrared detectors, for example, were made by wire bonding, flip-chip or beam-lead. In wire bonding, a gold or aluminum wire having a diameter of approximately 25 microns is attached to an electrode contact pad on the upper surface of a substrate by a thermal compression bond, a wedge bond, or an ultrasonic bond. The electrode contact pad is usually rectangular in shape and larger than 50 microns by 50 microns to permit the wire to be deformed without going off the contact and to permit a margin of operator error in the placement of the wire on the contact. The wire bonds were usually made between the surface of the component or integrated circuit and a supporting substrate having interconnecting metalization including a metalization contact pad for connection of the other end of the wire by bonding.
Flip-chip interconnections are usually formed by specially preparing the component to have raised contact pads which are faced downwards or positioned towards a substrate, which has matching contact pads; and all such component contact pads are simultaneously bonded to the substrate contact body by solder reflow or thermal compression bonding. Beam-lead interconnections are formed by using a specially prepared integrated circuit chip having cantilevered beams extending outwardly over the edge of the chip from the normal contact pad positions. The ends of the cantilevered beams are then bonded to metalization pads located on a supporting substrate. The plane of the beams may be aligned with the plane of the metalization of the substrate by insetting the components in the substrate or by inverting the components and having the beams connected to the substrate face down. The beams are bonded to the metalization of the substrate by thermal compression bonding or by ultrasonic bonding which may occur simultaneously for all beams or individually. Components which are interconnected on a common substrate or between themselves utilizing thin and thick film metalizations and wire, flip-chip or beam-lead bonding are referred to by those skilled in the art as hybrid circuits. The hybrid circuit is normally contained within a hermetically sealed package.
The advantages inherent in the present invention can be appreciated if we consider a particular application of this invention to the infrared focal plane assemblies. In infrared imaging systems, hundreds of detectors are fabricated on the focal plane. Conventional interconnecting techniques do not permit contact pads smaller than 50 microns in size. In such applications, each detector is approximately 30 microns .times. 30 microns in size with small detector to detector spacing. The small electrical contact size imposed by such detector size and the center-to-center spacing renders electrical interconnection cumbersome to couple directly the detectors on one component to signal processing circuitry on adjacent silicon integrated circuit chips.
Likewise, in the mounting of silicon integrated circuit (IC) chips in multiple chip hybrid packages (MHP) the number of bonding wires can be as high as a thousand, with a bond at each end. The probability of operator error is high when hand-bonded. In a machine bond operation, set-up and even operation time is long. On the other hand, bonding by the method taught by this invention would be quick, reliable and accurate. It would require only that the MHP substrate be mortised to receive the IC chips such that they be coplanar with the substrate.
In one particular embodiment in which this invention has been used, infrared detectors made of lead tin telluride were formed in an array. Each detector in the array was bonded by the electroplating method taught in this invention to its individual input contact in the integrated circuit (IC) which was formed on a silicon chip. Since the lead tin telluride is very soft conventional bonding techniques that require considerable pressure would have deformed or mechanically damaged the detection array. This invention has the advantage of not requiring any mechanical pressure so that it does not deform or damage the materials underneath the interconnections.
It is therefore desirable to be able to electrically interconnect on a small center-to-center spacing, the contacts of one component containing infrared detectors to contacts on another component containing signal processing circuitry such as charge coupled device shift registers, for example. It is desirable that the electrical interconnections between different circuit components may be made between contacts on the components having contact spacings of 100 microns or less. Furthermore, it is desirous that hundreds of interconnections may be made between two or more circuit components simultaneously.